The present invention relates generally to apparatus for coating a stripe of viscous material on a sealing surface.
Glass-to-glass seals, particularly useful in assembling various types of display tubes such as, for example, in assembling a glass face plate of a cathode ray tube to a funnel-shaped rear portion, are made by laying down a stripe of frit slurry containing low temperature melting glass particles in a binder in a closed pattern on a sealing surface of one of the two members to be sealed, placing the other member to be sealed against the coated sealing surface and heating the assembly to a temperature which causes the frit slurry to fuse and thereby to unite the two parts to be joined.
According to the prior art, a funnel portion of a cathode ray tube is positioned facing upward with its seal edge surface horizontally disposed. The funnel portion is rotated at a constant angular velocity and a nozzle is guided to remain aligned over the seal edge surface while a frit slurry is deposited from the nozzle on the seal edge surface to form a frit coating stripe. As is well known, the frit slurry used is quite viscous, having a viscosity suitably between about 5000 and about 20,000 centipoises and, in addition, is thixatropic. The binder of the frit slurry is also volatile. Deposition of the frit coating stripe may begin at an arbitrary point on the seal edge surface and continue in a closed pattern ending at a point coincident with the beginning. Since the beginning point of the frit coating stripe is coated some time before the nozzle completes its deposition at the end point (coinciding with the start point) the beginning of the frit coating stripe is hardened significantly and does not blend satisfactorily with the end portion of the frit coating stripe. Consequently, the end of the frit coating stripe overlays the beginning portion to create a bump. The extra material in the bump interferes with proper sealing and may be squeezed both outward and inward during the sealing operation. It is common in such sealing operations to employ a skilled manual worker to shape the overlapping portion in order to avoid this problem. The necessity for employing a skilled manual worker in this task prevents automation of the coating process and adds to the cost. Particles of frit slurry removed during the shaping operation can remain inside the tube after sealing and interfere with its operation. In addition, the binder used in frit slurry can be hazardous to the worker's health if he is not protected against it. Consequently, the health safety measures required to protect the skilled manual worker also add to the cost of the sealing operation.
One approach to solving the problem outlined in the preceding is disclosed in U.S. Pat. No. 4,137,341, issued Jan. 30, 1979, and having an assignee in common herewith. In this patent, a pair of nozzles begin a coating operation immediately adjacent to each other and follow opposite paths to end immediately adjacent to each other. This procedure has the advantage that the slurry from both nozzles has the same viscosity at the meeting points at the beginning of coating and also at the end. However, since the outlets of the nozzles are necessarily slightly separated from each other at both the start and end points, the frit slurry from the two nozzles may not always connect smoothly and continously to provide a frit coating stripe having uniform thickness and width. The outlets of the nozzles in this patent face generally in the downward direction. Independent means are thus required for starting and stopping the discharge of the frit slurry at the beginning and end points.
U.S. Pat. No. 3,947,311 shows an apparatus for sealing the edges of double-pane insulating glass sheets. In this patent, two horizontally disposed nozzles inject cement into the interspace between two glass sheets as the glass sheets are carried on the conveyor past the two nozzles. As noted, this patent is directed to sealing of the edges of double-pane insulating glass units and has no more than peripheral significance to the laying down of a continuous stripe of viscous material such as a frit slurry.